Thermal switch with temperature differential cycling delay



2 Sheets-Sheet 1 JZ ZZJVZWR EDMOND G. FRANKUN E. G. FRANKLIN THERMAL.SWITCH WITH TEMPERATURE DIFFERENTIAL CYCLING DELAY Oct. 30, 1956 FiledApril 30, 1954 E. G. FRANKLIN THERMAL SWITCH WITH TEMPERATUREDIFFERENTIAL CYCLING DELAY 2 Sheets-Sheet 2 Filed April 50, 1954[ZZYEJYZ'QR EDMOND G. FRANKLIN AYJ'Q ZJKEY Unite States Patent THERMALSWITCH WITH TEMPERATURE DIFFERENTIAL CYCLING DELAY Edmond G. Franklin,Minneapolis, Minn., assignor to General Mills, Inc., a corporation ofDelaware Application April 30, 1954, Serial No. 426,746

7 Claims. (Cl. 200-138) The present invention relates to thermalswitches, and more particularly to an improved thermal cycling delaymechanism for such a switch.

Thermal switches are well known in which the frequency of cycling issubstantially greater than is desired for particular applications. Forexample, in the home appliance field, there are certain standards suchas those of the National Electrical Manufacturers Association whichlimit the maximum number of cycles per unit time. In any case, arelatively high cycling frequency in a thermal switch is conducive tosubstantial wear and erosion of the contacts, which not only shortensthe effective life of the switch, but may contribute substantial errorsin the operating temperatures maintained by the switch.

To remedy these difiicnlties, some devices have been proposed in thepast in which one switch contact was supported by an auxiliarybimetallic strip having a cross section small enough to be heated by thecurrent passing through the contact. The heating of such a bimetallicstrip would then cause deflection of the strip to increase the contactpressure at the make and to increase the contact gap on cooling of thebimetal after the break. In these cases, however, to achieve thenecessary heating, the cross section of the auxiliary bimetallic striphad to be so small that the strip offered relatively little support forthe contact member.

With these problems of the prior art in view, it is one object of thepresent invention to provide a cycling delay mechanism for the thermalswitch in which one contact is supported by an auxiliary thermallyresponsive strip of substantial cross section.

Another obiect is the provision of such a switch in which a definitetemperature differential is maintained between the thermally responsivesupporting member for one contact and the contact supporting arm of theother contact.

A further object is a cycling delay arrangement for a thermal switch inwhich heat is conducted from one contact arm to the other uponengagement of the contacts, and in which this transfer of heat causesdeflection of the other contact in a direction toward the hottercontact.

Other objects and advantages of the invention will be apparent from thefollowing specification in which a preferred embodiment is described.

In the drawings which accompany this application and in which likereference characters indicate like parts,

Figure l is a side view with certain portions in section and otherportions broken away, illustrating one form of thermal switch embodyingthe present invention. In this figure the parts are shown in theposition occupied during the cooling portion of the cycle, just beforethe contacts re-engage each other;

Fig. 2 is a schematic view similar to Fig. 1 showing the parts at theinstant when the contacts first re-engage;

Fig. 3 is a view similar to Fig. 1 illustrating the position of thecontacts shortly after the engagement illustrated in 2,769,060 PatentedOct. 30, 1956 Fig. 2 has resulted in substantial heat transfer from onecontact to the other;

Figure 4 illustrates the position of the parts at the end of the heatingcycle just as the contacts disengage each other; and

Fig. 5 is a partial top view of the device of Fig. 1.

In the device shown in Figs. 1 through 5, the thermal switch isindicated generally at 20. For convenience in illustration, the switchhas been shown in combination with a plate 22 having an electricalheating element 24 associated therewith. The switch itself includes amain supporting bracket 26 with horizontal flanges 28 at its loweredges. These horizontal flanges are secured to appropniate portions ofthe heated plate 22 by bolts 30. This bracket preferably should be apoor heat conductor such as stainless steel.

Bracket 26 includes a horizontal top wall portion 32 on which adepending stud or post 34 is carried. Post 34 in turn supports thevarious switch elements. Immediately below the top 32 of bracket 26,post 34 carries an insulating block 36. Beneath this block is securedthe base portion 38 of an upper contact arm 40. Next below the contactarm portion 38 is another insulating member 42, and below that issecured the base portion 44 of a shielding or heat insulating member 46.Another insulating block 48 spaces the base portion 50 of a lowercontact arm 52 below the heat shield. An insulator 54 and retainingplate 56 are secured to the bottom of post 34 to complete the assembly.

The base portion 38 of the upper cont-act arm includes a terminalextension 58, while the base portion 50 of the lower contact armincludes a similar terminal 60. Wires 62 and 64 are connected to therespective terminals 58 and 60 and serve to connect the heating element24 in circuit with the switch in known manner.

The upper Wall 32 of supporting bracket 26 is provided with a threadedboss 66 which receives the threaded end 68 of a manually adjustablecontrol member 70. An insulating button 72 at the lower end of controlshaft engages the outer end 74 of flexible contact arm 40. This arm isnormally resiliently bias-ed upwardly so that end portion 74 is heldagainst insulating button 72 at all times. Thus, rotation of shaft 70changes the vertical position of the insulating button and predeterminesthe operating position of the upper contact arm 40.

The upper contact 76 is supported from contact arm 40 by means of anauxiliary thermally responsive member. For purposes of illustration lhave shown a bimetallic strip 78. This strip extends generallyhorizontally parallel to and slightly below the outer end 74 of thecontact arm. At its inner end, the bimetallic strip 78 is offsetupwardly at 8-0 and has its end portion 82 riveted at 84 to the contactarm. This point of attachment is so chosen that flexing of the contactarm 40 in response to adjustment of shaft 70 will cause correspondingchanges in the initial position of contact 76. The relative orientationof the high and low expansion sides of the bimetallic strip 78 is suchthat upon heating of the strip the upper contact 76 will be deflecteddownwardly toward its opposing contact 86.

This lower contact 86 is carried at the outer end 88 of the lowercontact arm 52. The lower contact arm 52 is resiliently biased upwardlyso that contact 86 is normally urged up toward its cooperating contact76. The actual position of this lower contact is controlled in responseto changes in the thermal condition to be controlled. For

this purpose, and solely by way of illustration, a connection isprovided by a rivet 90 between the lower contact arm and a bracketmember 94. This bracket member 94 has a horizontal flange 92 at itsupper end which is electrically insulated from the contact arm 52 byinsulating members 96. The parts are held firmly in assembled relationby the above-mentioned rivet.

The lower end of bracket 94 includes a horizontal flange 98 whichcarries a connecting pin or rivet 100 for attachment of one end 192 of athermal control member 104. The other end of member 104 is connected at106 to a remote portion of the heating plate 22 by means of a bolt 110threaded into a projection 108 on the plate.

The material from which the member 104 is made preferably has asubstantially smaller coefficient of thermal expansion than the heatingplate 22 itself. Thus the relative difference in expansion between theplate 22 and member 104 over the distance between switch securing bolts36 and the control retaining bolt 110 provides a thermally responsiveunit which effectively raises and lowers the lower contact arm 52. Sincethe lower contact 86 is normally biased upwardly, it will be apparentthat the linkage will involve no lost motion and that the control member104 will at all times be under tension to hold the contact arm 52downwardly to a greater or lesser extent against the upward bias of arm52.

Instead of the difference in expansion between member 104 and plate 22,other thermal controls of known type may be used to actuate the lowercontact arm 88 of the switch. For example, a bimetallic strip responsiveto temperature of plate 22 could be connected in known manner to movethe lower contact arm and operate the switch in response to desiredpredetermined plate temperatures.

The heat insulating shield 46 described above projects outwardly to apoint 112 substantially close to the coopcrating contacts 76 and 86.This shielding member is wider than the auxiliary bimetallic strip 78 ofthe upper contact. As shown in the drawing, the shield member 46 liesbetween this bimetal strip and the lower contact arm 52 throughout amajor area of the bimetal. Thus the bimetallic strip 78 is effectivelyshielded against transfer of heat by radiation from the lower contactarm 52 or from the heating plate 22 and associated heating element 24.End 112 of the shield may serve as a limiting stop for the uppercontact, as shown in Figs. 3 and 4.

On the otherhand, the lower contact arm 52 and its contact 86 are notshielded in this respect and are so located thatthey will normally havea substantially higher temperature than the shielded bimetallic stripportion 78, whenever contacts 76 and 86 are separated.

According to the present invention contacts 76 and 86 are made of silveror some alloyiof very high heat-conducting properties. Thus theengagement of thesev contacts not only completes the electrical circuitbetween the terminals 58 and 60, but also provides a good heatconductingpath from the lower contact arm'at 88 to the bimetallic supporting strip78.

. With these details of construction in mind, the operation of theswitch shown in the drawings will be readily understood. Fig. 1illustrates the position of the parts during the cooling portion of thecycle while the contacts are disengaged and the lower contact isgradually moving up toward the upper contact to re-establish the heatingcycle. During this part of the cycle the lower contact 86 and contactarm 88 will reach a substantially higher equilibrium temperature thanthe upper contact supporting portion 78, both because of the relativenearness of the 'lower arm to the heating plate 22 and also because ofthe effect of the intervening heat shield 46. Because of the shieldingeffect, the bimetallic supporting member 78 will occupy a normalposition which is substantially undefiected.

7 On further upward movement of the lower contact, as illustrated inFig. 2, the contacts 76 and 86 will re-engage each other to establish afurther heating cycle. Fig. 2 illustrates the position of these barsjust at the instant of re-engagement, before there is any substantialheat trans fer from the lower contact arm to the upper one. Fig. 3illustrates the condition of the parts a moment later, after therelative difference in temperature of the lower and upper contact armshas been equalized by rapid conduc tion through the contacts themselves.The resulting increase of temperature of the upper contact arm causes itto be deflected downwardly from the position of Fig. 2 to the positionof Fig. 3.

This downward deflection has been exaggerated in the drawing forclearness, although in a practical case, the amount of such deflectionwill depend to some extent upon the relative rigidity of the lowercontact arm at 88. If this lower contact arm is relatively rigid, therewill be little deflection, but merely an increase in contact ressure bythe upper arm. Whichever situation results, however, it will benecessary for the lower contact to move downwardly a greater distancethan would otherwise be the case, in order to separate the contacts oncemore. Thus the heating portion of the cycle is effectively lengthened.

It should be noted at this point that the relative crosssection of thebimetallic supporting strip 78 may be great enough so that nosubstantial heating and deflection of the bimetal will occur solely bypassage of current to the contact. In such a case the primary cause ofdeflection is the increase and decrease of temperature in the bimetallicstrip resulting from engagement and disengage ment of the contacts andthe substantial differences in temperature at which the respectivecontact arms are maintained. In the preferred form of the invention,however, both effects are used cumulatively. The deflection of thecontact support, or the change in contact pressure, is then due both tothe heating of the thermal member by current passing to the contact andto the conduction of heat from the other contact. Thus the cross-sectionof the thermally responsive cycling delay member can be great enough toprovide a firm support for the contact, rather than the weak andflexible support involved in some prior art constructions where thedeflection of the contact is achieved solely by heating a bimetalthrough passage of current to the contact.

As the heating cycle continues, the lower contact will gradually movedownwardly until it reaches the position of Fig. 4 at which the contactshave just begun to disengage each other. As soon as the contactsdisengage, the heat transfer path from one to the other is interrupted,and the respective contact arms are free to resume their differentequilibrium temperatures as before. Thus there is no further heattransfer from the lower contact arm to the upper one and the uppercontact arm will begin to cool and be deflected back upwardly toward theposition of Figs. 1 and 2. This upward movement increases the contactseparation and thus increases the time required before cooling of theplate 22 can result in sufficient movement of contact 86 to re-establishthe circuit.

The foregoing cycle will be repeated at each make and break of thecontacts and will thus achieve the desired cycling delay, primarily as aresult of maintenance of a substantial temperature differential betweenthe Zones in which the upper and lower contact arms are located and theconduction of heat directly from one contact arm to the other as thecontacts engage. The relative temperature differential is enhanced bythe particular location and arrangement of the shielding member 46.

According to the foregoing description a thermal switch has beenprovided which substantially accomplishes the objects set forth at thebeginning of this application. It should be noted that a thermaldifferential cycling delay of the type described herein is mosteffective in applications where a relatively great temperaturedifferential can be readily obtained between the region of the lowercontact arm and the area in which the upper contact arm is located.Also, while the thermally responsive contact support has been shown inthe form of a bimetal strip, there may be situations in which athermally responsive expanding and contracting member may offer addedadvantages. Such members are shown, for example, in my earlier copendingapplication Serial No. 309,325, filed SeptemberlZ; 1952, now Patent No.2,716,172, and in the copending application of Franklin and Tsai,

Serial No. 309,326, filed September 12, 1952, now Patent No. 2,716,174.The invention also contemplates the alternate possibility ofrearrangement of parts for removal of heat from the thermal portion byconduction through the contacts to achieve the desired results.

Since minor variations and changes in the exact details of constructionwill be apparent to persons skilled in this field, it is intended thatthis invention shall cover all such changes and modifications as fallwithin the spirit and scope of the attached claims.

I claim as my invention:

1. A thermal switch comprising a first contact arm and a first contactcarried by said arm, first thermally responsive means on the first armhaving direct thermal engagement with the first contact and at leastpartially controlling the relative position of the first contact, asecond contact, second thermally responsive means for causing relativeengagement and disengagement of the contacts in response topredetermined changes in temperature of said second means, and meansnormally maintaining a substantial temperature differential between thesecond contact on the one hand, and the first contact and firstthermally responsive means on the other hand when the contacts aredisengaged, said contacts having a relatively high thermal conductivitysuch that engagement of the contacts causes heat transfer by conductionthrough the contacts thereby changing the temperature of the firstthermally responsive means and urging the first contact toward thesecond in response to said temperature change by conduction.

2. A thermal switch according to claim 1 in which the first thermallyresponsive means is in electrical circuit with one contact and has across section such that passage of current through the contacts whenthey engage each other causes surficient heating of the first thermallyresponsive means to increase the urging of the first contact toward thesecond in response to engagement of the contacts.

3. A thermal switch comprising first and second movable contact arms, abimetallic strip secured to the first arm and supporting a firstcontact, and a second contact secured to the second arm, each of saidfirst and second contacts being of high thermal conductivity and indirect heat conducting engagement with the bimetallic strip and secondcontact arm respectively, and means maintaining a substantialtemperature differential between the second contact arm and thebimetallic strip when the contacts are disengaged, the relatively highthermal conductivity of said contacts causing heat transfer byconduction through the contacts when they engage each other and therebychanging the temperature of the bimetallic strip, said strip beingoriented and connected to urge the first contact toward the second inresponse to said temperature change by conduction.

4. A thermal switch according to claim 3 in which said temperaturedifferential maintaining means includes a heat insulating shield betweenthe bimetallic strip and second contact arm.

5. A thermal switch for use in combination with a heated member and aheating element for said member, said switch comprising first and secondmovable contact arms for connection to control said heating element, a

bimetallic strip secured to the first contact arm and extendinggenerally parallel thereto, a first contact sup ported by said strip,and a second contact supported by said second contact arm, supportingmeans for said contact arms adapted to support the second contact arm ina first temperature and thereby maintain said arm at a given ambienttemperature, said supporting means also being adapted to carry the firstcontact arm and bimetallic strip in a second zone of substantiallydiiferent temperature, thereby maintaining the bimetallic strip at asubstantially different ambient temperature from the second contact arm,said contacts having a relatively high thermal conductivity such thatengagement thereof causes substantial heat transfer between the secondcontact arm and the bimetallic strip, thereby deflecting the latter in adirection urging the first contact toward the second, and thermallyresponsive means operative y connected to one of said contact arms andmoving the contacts into and out of engagement in response topredetermined variations in the temperature of the heated member.

6. A thermal switch for use in combination with a heated member and aheating element for said member, said switch comprising first and secondmovable contact arms for connection to control said heating element, abimetallic strip secured to the first contact arm and extendinggenerally parallel thereto, a first contact supported by said strip, anda second contact supported by said second contact arm, supporting meansfor said contact arms adapted to support the second contact armrelatively close to the heated member and thereby maintain said arm at ahigh ambient temperature, said supporting means also being adapted tocarry the first contact arm and bimetallic strip relatively farther fromthe heated member, a heat-insulating shield carried by said supportingmeans and located between the bimetallic strip and second contact arm,thereby maintaining the bimetallic strip at a substantially lowerambient temperature than the second contact arm, said contacts having arelatively high thermal conductivity such that engagement thereof causessubstantial heat transfer from the second contact arm to the bimetallicstrip, thereby heating and deflecting the latter in a direction urgingthe first contact toward the second, and thermally responsive meansoperatively connected to one of said contact arms and moving thecontacts into and out of engagement in response to predeterminedvariations in the temperature of the heated member.

7. A thermal switch according to claim 6 in which the thermallyresponsive means is operatively connected to the second contact arm, andmanually adjustable control means operatively connected to the firstcontact arm, adjustment of said control means changing the position ofthe first contact arm and thereby determining the operating temperatureof the switch.

References Cited in the file of this patent UNITED STATES PATENTS1,959,205 Hanel May 15, 1934 2,208,432 Samuels July 16, 1940 2,667,565Wallower Jan. 26, 1954

